The present invention relates to an off-mains switching system for automatically switching off the main voltage to current consuming devices such as actuators in response to these being switched off.
In different installations, e.g. in connection with an actuator equipment on adjustable hospital beds, it is, for reasons of security, desirable that the main voltage to the equipment be kept interrupted whenever there is no actual need for it. Such an interruption or off-mains switching may be desirable also for other reasons, e.g. for avoiding electrical fields in bedrooms or for saving energy consumption in devices with idling losses, for example transformers. A superior interruption could be arranged by means of a manually operated main switch for the single equipment or section of installation, but of course it is to be preferred that the off-mains switching can happen automatically as a function of an operational switching off of the unit, without any need of remembering to then also operate a main switch. Correspondingly, a detector should be able to react on a new attempt to switch on the unit, such that the current supply can be reestablished as the need arises, without a main switch first having to be operated.
At the principal level it is the latter function which is the more problematic, because an earlier interruption of the voltage supply will imply that even the said detector is without supply. It is already known, however, that this problem can be dealt with by the use of a battery in the detector unit, see e.g. DE-A-2624316 and WO 93/11593, such that the voltage available for the detection of a desired restart of the operative unit will be in an acceptable low voltage range.
It is even known that the said battery can be backed up by a condenser voltage, which is maintained as long as the operative unit is subjected to the mains, and which may remain operative through a relatively long time after the last switching off of the unit, when a condenser with a high discharge resistance is used.
On this background it could be possible to avoid the battery, viz. if the equipment is operated daily or even just once a week, as a sufficient condenser voltage may well be maintained through some weeks after each charging. If the equipment is not used for longer time, a renewed start can be arranged by way of current supply from an external, mobile battery, or by actuating the equipment by means of a parallel connection directly to the mains, using an additional cable and a switch. For different reasons, however, none of these solutions are particularly attractive.